Isolated gum operon from Xyllela fastidiosa, isolated nucleic acid molecules therefrom, and uses thereof

ABSTRACT

This invention relates to the isolation and characterization of an operon from the organism  Xylella fastidiosa . This operon, referred to as the XfGUM operon, contains 9 genes, and is involved in the production of a compound with xanthan gum like properties. The expression products of the operon and genes are also a part of the invention, as are various uses of these.

This invention was made with FAPESP support.

FIELD OF THE INVENTION

This invention relates to the isolation of an operon of the microorganism Xylella fastidiosa. More particularly, it relates to the isolation of such an operon which comprises nine genes, referred to as “xfgum B, C, D, E, F, H, J, K and M” hereafter. Also a part of the invention are isolated nucleic acid molecules for each of these 9 separate genes. Also a part of the invention are expression vectors which comprise the operon, or one or more of the component genes, in operable linkage with a promoter, recombinant cells which include the operon or one or more of the genes, the expression product of these genes, and various uses thereof.

BACKGROUND AND PRIOR ART

Xanthan gum is an exopolysaccharide (“EPS” hereafter) produced by the phytopathogenic bacterium Xanthomonas campestris (Ielpi et al., 1981, FEBS Lett. 130:253-256). The biosynthesis of this extracellular polysaccharide in X campestris is directed by a cluster of 12 genes, gumB-gumM (Becker et al., 1998, Appl Microbiol Biotechnol, 50:145-152; Vojnov et al., 1998, Microbiology 144; 487-493). It is composed of polymerized pentasaccharide repeating units which are assembled by the sequential addition of glucose-1-phosphate, glucose, mannose, glucuronic acid, and mannose on a polyprenol phosphate carrier (Ielpi et al., 1993, J. Bacteriol. 175:2490-2500). The pentasaccharide repeating unit is also O-acetylated and pyruvylated to various degrees (Ielpi et al., 1993, J. Bacteriol. 175:2490-2500).

Transformation with a gene cluster carrying gumB and gumC restored xanthan production in deficient mutants, suggesting that gumB and gumC are both involved in the translocation of xanthan across the bacterial membrane (Vojnov et al., 1998, Microbiology 144: 487-493).

Two gene groups are involved in xanthan biosynthesis: the genes xpsIII, xps IV and xps VI, which are responsible for synthesis of UDP-glucose, UDP-glucuronic acid and GDP-mannose, respectively (Harding et al., J. Bacteriol 169: 2854-2861 (1987); Harding et al., J. Gen Micobiol 139: 447-457 (1993); Hötte et al., 1999; Köplin et al. J. Bacteriol 174: 191-199 (1992), 1992) and the genes gum or xpsI, which encode the enzymes involved in the polymerization of the pentasaccharide linked to polyprenol phosphate and secretion of the EPS to the extracellular medium (Ielpi et al., 1992; supra Ielpi et al., 1993 supra).

The X. campestris gum genes are grouped in an operon (GUM operon) of about 16 Kb in length, containing 12 genes designated gumB, C, D, E, F, G, H, I, J, K, L and M. The gumD gene encodes the enzyme glucosyltranferase I, responsible for the transference of the glucose-1-phosphate of UDP-glucose to the polyprenol phosphate carrier to form the lipid-monosaccharide. GumM encodes the enzyme glucosyltranferase II, which catalyzes the addition of the second glucose residue to form the lipid-disaccharide. GumH encodes the enzyme glucosyltranferase III, which catalyzes the addition of the first mannose residue to form the lipid-trisaccharide. GumK encodes the enzyme glucosyltranferase IV, which catalyzes the addition of a glucuronic acid residue to form the lipid-tetrasaccharide. GumI encodes the enzyme glucosyltranferase V, which catalyzes the addition of a second mannose residue to form the lipid-pentasaccharide. GumF encodes the enzyme acetyltransferase I which catalyzes acetylation of the internal mannose residue. GumG encodes the enzyme acetyltransferase II which catalyzes the acetylation of the external mannose residue. Finally, gumL encodes the enzyme pyruvate ketal transferase which catalyzes pyruvylation of the external mannose residue. GumB, C and E are involved in the polymerization and secretion of the EPS through the bacterial membrane while gumJ is thought to be involved in a parallel stage of polymerization and secretion of the EPS (Ielpi et al., 1992 supra; Ielpi et al., 1993 supra).

Xanthan has wide commercial application as a viscosifier of aqueous solutions. It is used in both the food and non-food industries and also as a stabilizer of suspensions, emulsions, and foams. Mutant Xanthomonas strains defective in the xanthan biosynthesis pathway have been shown to synthesize and polymerize xanthans with variant structures, similar rheological properties, and different viscosities (Hassler and Doherty, 1990, Biotechnical Prog 6:182-187). Acetylation and pyruvylation can affect the viscometric properties of xanthan. The presence of pyruvate increases viscosity, whereas acetate decreases viscosity (Hassler and Doherty, supra). The elimination of sugar residues from xanthan side chains also affects viscosity. As compared to wild-type xanthan, polymers lacking the terminal mannose (polytetramers) are poor viscosifiers. In contrast, polymers lacking both the terminal mannose and glucuronic acid residues (polytrimers) are superior viscosifiers (Hassler and Doherty, supra). A nonacetylated and an acetylated tetramer, both lacking the side-chain terminal mannose residue and in the first case lacking an acetate group on an internal mannose residue showed higher viscosity (in the first case) and lower viscosity (in the second case) when compared to a wild-type bacteria (Levy et al., 1996, Biopolymers 38:251-272). A major conformational difference between the higher and lower viscosities is the increased amount of open helical backbone and the increase in the side-chain flexibility high viscosity (Levy et al., 1996, Biopolymers 38:251-272).

Xanthan has also been used as a drug stabilizer, to improve drug absorption, to delay drug release and can also be used in controlled-release formulations (Talukdar et al., 1996, J Pharm Sci 185:537-540). In combination with xanthan gum, alpha-cyclodextrin reduced the first-pass metabolism of morphine in the rectal mucosa and by the liver, and was shown to improve the apparent rectal bioavailability of the opioid about 4 fold (Kondo et al., 1996, Biol Pharm Bull 19:280-286). Xanthan-alginate has been used for encapsulation of enzymes. The xanthan-alginate spheres showed 75% of maximum urease activity even after 20 repeated uses under optimal conditions (Elgin, 1995, Biomaterials 16:1157-1161).

Sustained-release hydrogel suppositories prepared with water-soluble dietary fibers, xanthan gum and locust bean gum have been shown to sustain drug release for a much longer time than commercial suppositories. The mean residence time was higher, without a decrease in the area under the plasma concentration vs. time curve. Histopathological studies showed good biological safety of the hydrogel suppositories to the rectal mucosa. These results suggested that IMC hydrogel suppositories prepared with xanthan gum and locust bean gum were a practical rectal preparation having prolonged action and reduced side effects (Watanabe et al., 1993, Bio Pharm Bull 16:391-394).

The apparent release rate of prednisolone from hydrogels prepared with xanthan decreased with increasing gum concentration, suggestion that the diffusion of drug molecules was mainly controlled by the density of the three-dimensional network structure in the matrix. These results indicated that drug release could be controlled not only by the density of the network structure but also by the microscopic viscosity of the hydrogels (Watanabe et al., 1992, Chem Pharm Bull 40:459-462).

A neomycin-furazolidone-xanthan complex has been shown to increase the antimicrobial activity of the drug (Dumitriu et al., 1993, J. Biomater 7:256-276).

Xanthan gum has also been shown to alleviate diabetes mellitus. Administration of xanthan gum lowered fasting and postload serum glucose and reduced fasting levels of total plasma cholesterol in diabetic subjects. Xanthan gum also tended to lower fasting and postload levels of gastrin and gastric inhibitory polypeptide (GIP) and fasting levels of total and VLDL triglyceride and cholesterol in VLDL and LDL fractions. Subjects reported a sense of fullness after consuming xanthan muffins but no severe digestive symptoms (Osilesi et al., 1985, Am J Clin Nutr 42:597-603).

Several factors have been associated with pathogenicity of phytopathogenic bacteria, including production of extracellular enzymes, production of exopolysaccharides, production of toxins and phytohormones along with factors mediating specific plant pathogen interactions (Agrios, 1988, Plant Pathology, 3rd edition, Academic Press; Alippi, 1992, Agronomie 12:115-122). The role of EPSs in the pathogenicity of several bacteria is well known. They are associated with (i) cell death due to occlusion of xylem vessels, (ii) mucoid lesions and (iii) helping plant/pathogen interaction and bacterial growth in the plant tissues.

Several groups have investigated the role of xanthan in Xanthomonas pathogenicity. Although it was believed for a long time that xanthan gum is key to pathogenicity, many authors did not succeed in demonstrating a direct link between this EPS and pathogenicity (Kamoun and Kado, 1990, J. Bacteriol 172:5165-5172). Recently, it has been demonstrated that deletion of gumD (Chou et al., 1997, Biochem Biophys Res. Commun 233:265-269) or alterations in the later stages of xanthan biosynthesis (Katzen et al., 1998, J. Bacteriol 180:1607-1617) reduced virulence in causing black rot in broccoli and decreased the aggressiveness of Xanthomonas against plants. Although the symptoms of X. fastidiosa infection of orange trees are well known, e.g., leaf clorosis, nutritional deficiency symptoms, reduced fruit size, etc. (De Negri, 1990, Com. Tec. No. 82, Ext. Rural, Cats, Compinas; Malavolta et al., 1990, Cordeiropolis, v. 11, n. 1, p. 15-18), the mechanism by which the bacteria causes the disease is unknown. Bacterial infection depends on insect vectors such as Oncometopia facialis, Acrogonia terminalis and Dilobopterus costalimai (Gravena, et al., 1997, “Os vetores de Xylella fastidiosa In: Clorose Variegada do Citros”). These insects, when feeding on xylem nutrients, introduce the bacteria into the vessel where bacteria grows fastidiously (Lopes et al., 1996, “Xylella fastidiosa,” Fitopathologin Brasileira, Brasilia v. 21, Suplemento p. 343; Roberto et al., 1996, Fitopatologia Brasileira, Brasilia v. 21, n. 4, p. 517-18). The EPS synthesized by the XfGUM operon, which is a feature of the invention, might be directly involved in the pathogenicity causing Citrus Variegated Clorosis in orange trees and other diseases in plants, such as coffee and grape plants.

SUMMARY OF THE INVENTION

The invention relates, in part, to a nucleic acid molecule of about 12710 nucleotides which encodes the members of the XfGUM operon, including genes for xfgumB, xfgumC, xfgumD, xfgumE, xfgumF, xfgumH, xfgumJ, xfgumK and xfgumM and the uses of such operons, as well as the proteins encoded thereby. These can be used, e.g., to control orange tree diseases, such as Citrus Variegated Clorosis (CVC) and diseases caused by the bacteria in other plants. The EPS produced by the enzymes encoded by the XfGUM operon may cause the diseases by occluding the xylem vessels, or by participating in the mechanism of bacteria infection, adhesion and spreading; however, applicants do not limit themselves to any single theory of operation. The invention also relates to the use of components of the XfGUM operon for production of exopolysaccharides, useful industrial and/or medical application of this new gum and structural gums variants, using Xylella fastidiosa or any other bacteria as a host.

The invention is based in a number of unanticipated and surprising discoveries. One is the discovery of a DNA sequence of 12710 nucleotides in the genome of the phytopathogenic bacteria Xylella fastidiosa. A xanthan GUM operon has been found previously only in Xanthomonas campestris. Another is the finding that the XfGUM operon lacks genes gumI, gumG and gumL, which encode the enzymes glucosyltransferase V, acetyltranferase II and pyruvate ketal transferase, respectively. The absence of these genes indicate that the XfGUM operon encodes the enzymes necessary to synthesize an exopolysaccharide composed of tetrameric units consisted of glucose, glucose, mannose and glucuronic acid and that, in view of the absence of the genes encoding the acetyltransferase II and pyruvate ketal transferase, the resulting tetrasaccharide is not pyruvylated at all and, as demonstrated for Xanthomonas, its mannose may be only partially acetylated. These findings indicate that XfGUM encodes the enzymes that produce a new gum with new Theological and chemical properties.

The invention includes, inter alia, (a) nucleotide sequences containing the structural and regulatory regions of the XfGUM operon that encode the enzymes glycosyltranferase I, glycosyltranferase II, glycosyltranferase III, glycosyltranferase IV, and acetyltransferase I, as well as the proteins referred to hereafter as GUMB, GUMC, GUME and GUMJ; (b) the expression of such nucleotide sequences in cells; (c) the isolated enzymes, proteins, and active forms thereof, as well as mutants, fragments, and fusion proteins; (d) the uses of the enzymes and/or proteins encoded by the GUM-like Operon in screening methods for identifying compounds that are useful in controlling diseases caused by Xylella fastidiosa and/or related organisms; (e) the engineering of bacterial hosts to express the enzymes and/or proteins encoded by XfGUM; and (f) the use of the engineered hosts expressing the enzymes and/or proteins encoded by XfGUM to produce the exopolysaccharide for industrial and/or medical applications, and for animal use, including humans.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A to 1I present Xanthomonas campestris Gum operon sequences of the Prior Art.

FIG. 1A presents the amino acid sequence of Xanthomonas campestris protein GumB (SEQ ID NO: 20).

FIG. 1B presents the amino acid sequence of Xanthomonas campestris protein GumC (SEQ ID NO: 21).

FIG. 1C presents the amino acid sequence of Xanthomonas campestris protein GumD (SEQ ID NO: 22).

FIG. 1D presents the amino acid sequence of Xanthomonas campestris protein GumE (SEQ ID NO: 23).

FIG. 1E presents the amino acid sequence of Xanthomonas campestris protein GumF (SEQ ID NO: 24).

FIG. 1F presents the amino acid sequence of Xanthomonas campestris protein GumH (SEQ ID NO: 25).

FIG. 1G presents the amino acid sequence of Xanthomonas campestris protein GumJ (SEQ ID NO: 26).

FIG. 1H presents the amino acid sequence of Xanthomonas campestris protein GumK (SEQ ID NO: 27).

FIG. 1 i presents the amino acid sequence of Xanthomonas campestris protein GumM (SEQ ID NO: 28).

FIG. 2 sets forth a proposed biosynthetic pathway using the components of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a nucleic acid molecule of about 12710 nucleotides of the Xylella fastidiosa genome that encodes the operon XfGUM comprising genes xfgumB, xfgumC, xfgumD, xfgumE, xfgumF, xfgumH, xfgumJ, xfgumK and xfgumM and the uses of such operon, either as DNA as well as the proteins it encodes, for the control of the orange tree disease Citrus Variegated Clorosis (CVC) or related diseases caused by the bacteria in other plants, such as oak trees and grapevines.

The following definition is provided to remove any potential ambiguities as to the intent or scope of their usage in the specification and claims.

“Similarity” as used herein refers to identity of nucleotide sequences. A sequence is said to be 70% similar to another one if, after a global alignment, 70% of the nucleotides of the smaller sequence are identical to the corresponding nucleotides of the other sequence. Global alignments are done by means of dynamic programming (Thompson J D et al. (1997). Nucleic Acids Res. 25, 4876-4882 incorporated by reference). This guarantees a mathematically optimal alignment assigning a value for matches and mismatches between all nucleotides and penalties for insertions or deletions of different lengths. Typical numbers are a score of 1.9 for a match, 0 for a mismatch, for opening a gap and 6.66 for extending this gap.

EXAMPLE 1

A cosmid library was prepared from total DNA of Xylella fastidiosa, using standard techniques, including the digestion of the DNA with restriction enzyme BamHI. Those fragments which ranged from about 35-45 kilobases, were selected as being most relevant. A hybridization map was prepared, and reviewed to select the cosmids which did not overlap. Following selection, non-overlapping cosmids were sequenced using the art recognized shotgun approach. One cosmid was selected, in view of similarity to Xanthomonas spp. sequences. A fragment of about 12710 nucleotides was isolated, and studied further.

EXAMPLE 2

The further studies referred to in example 1, supra, included comparison of the 12710 nucleotide fragment to the known GUM operon of Xanthomonas campestris. The known operon has been described as containing 12 genes; however, the operon disclosed herein appears to contain only nine, i.e., it contains genes with some homology to gumB, C, D, E, F, H, J, K and M; however, no sequences homologous to gumG, I, or L were found.

In order to distinguish the two sets of genes, those which are a part of the invention will be described hereafter as “xfgum” genes, and the operon will be referred to as “XfGUM.”

The nucleotide sequences of xfgumB, C, D, E, F, H, J, K and M are set forth as SEQ ID NOS:1-9, respectively. The complete XfGUM operon is set forth at SEQ ID NO:110.

The putative amino acid sequences encoded by the referenced xfgum genes are set forth as SEQ ID NOS:11-19. These were compared to the comparable amino acid sequences for Xanthomonas campestris. In FIGS. 1A-1I, homology analysis is presented, using the abbreviations “GUMBXf, GUMCXf,” etc., for the proteins of the invention, and “GUMBXc, GUMCXc,” etc., for the X. campestris proteins.

EXAMPLE 3

The homology of the sequences from Xylella fastidiosa to the Xanthomonas campestris sequences led to conclusions about the functions of the encoded proteins, and a proposed mechanism for the synthesis of the xanthan-like product by Xylella fastidiosa, which is presented in FIG. 2. Specifically, xfgumD encodes a glycosyltransferase I homologue, involved in addition of a first glucose residue via transfer of glucose-1-phosphate from UDP-glucose to an isoprenol phosphate carrier, resulting in the lipid monosaccharide. The next step in the synthesis involves the addition of a second glucose residue via a glycosyltransferase II homologue, encoded by xfgumM, to form a lipid disaccharide. Following this, a glycosyltransferase III homologue encoded by xfgumH is believed to add a mannose residue to form the trisaccharide, after which a glycosyltransferase IV homologue encoded by xfgumK adds a glucouronic acid residue, forming the tetrasaccharide. The xfgumF gene encodes an acetyltransferase I homologue, which catalyzes acetylation of the mannose reside referred to supra. The proteins encoded by xfgumB, C and E appear to be involved in secretion of the xanthan-like EPS through bacterial membranes, while the protein encoded by xfgumJ appears to be involved in the polymerization and secretion of the xanthan-like EPS.

As noted supra, the XfGUM operon lacks components corresponding to Xanthomonas syp. genes GUMG, I and L, which encode acetyltransferase II, glucosyltransferase V, and pyruvate transferase. This suggests that the EPS produced by Xylella fastidiosa consists of tetrameric units of glucose-glucose-mannose-glucuronic acid, where these tetramers are not pyruvylated, and are acetylated in whole or part.

Hassler, et al., Biotechnol Proj. 6:182-187 (1990), and Levy, et al., Biopolymers 38:251-272 (1996), both indicate that mutant strains of Xanthanomas, which are defective in one or more steps of the xanthan biosynthetic pathway synthesize structural variants of xanthans, which had chemical properties that differ from each other and standard xanthans. Both acetylation and pyruvylation are known as processes which affect viscometric properties of xanthan. Pyruvate increases viscosity, and acetate decreases it. Further, when terminal mannose is absent, viscosity decreases. Nonacetylated tetramers lacking side chain terminal mannose have higher viscosity. These prior observations suggest that the EPS described herein will have properties that differ from previously identified xanthans.

The foregoing description, including examples, describes features of the invention. These include isolated nucleic acid molecules which encode any and all of the xfgum proteins described herein, including GUMxfB, C, D, E, F, H, J, K and M, as well as the proteins themselves. Especially preferred are those isolated nucleic acid molecules which comprise or consist of the nucleotide sequences of SEQ ID NOS: 1-9, and nucleic acid molecules which differ therefrom but, in view of the degeneracy of the genetic code, still encode the proteins which have the amino acid sequences set forth in SEQ ID NOS: 11-19.

The isolated nucleic acid molecules of the invention may be included in expression vectors, such that the nucleic acid molecules are in operable linkage with a promoter. In accordance with this feature of the invention, more than one of the nucleic acid molecules described and claimed herein may be included in the expression vector. There are nine nucleic acid molecule families described herein. Any and all combinations of these may be a part of these expression vectors. In other words, combinations of 2, 3, 4, 5, 6, 7, and 8 of the nucleic acid molecules may be a part of these vectors, as well as expression vectors including all 9, or just 1.

Also a part of the invention are recombinant cells, such as recombinant prokaryotic cell strains, and recombinant eukaryotic cell lines, that have been transformed or transfected with one or more of the isolated nucleic acid molecules or recombinant expression vectors of the invention.

Also a part of the invention, as described herein, is the isolated XfGUM operon. A preferred embodiment is that set out in SEQ ID NO: 10; however, it will be understood that the statements regarding the degeneracy of the genetic code referred to supra apply here as well. Further, in connection with the operon, as well as all nucleic acid molecules described herein, principles of preferred codon usage apply in connection with the use of the nucleic acid molecules for transforming or transfecting cells.

As indicated, supra, also a feature of the invention are the proteins encoded by the isolated nucleic acid molecules. These proteins have enzymatic activity, as described herein. Further, they can be used as immunogens to generate antibodies, both polyclonal and monoclonal, which can be used as described infra. Further, these proteins can be produced in various recombinant forms, including fusion proteins, in active forms that consist of less than the entire amino acid sequence of the protein, and so forth. For example, it is well known that enzymes are composed of various structural domains, including one or more active sites. Such active sites are a part of the invention, and comprise less than the entire amino acid sequence of the protein.

As indicated, the proteins of the invention can be used to prepare xanthan like molecules, such as those illustrated in FIG. 2, i.e.:

-   -   [glucose-glucose-mannose-glucuronic acid]n.         where n is a whole number from 1 to about 5000, preferably from         about 300 to about 5000, most preferably from about 1500-3000.

This structure may be acetylated; however, as indicated supra, when made via a process that comprises the use of the enzymes that are a part of this invention, the resulting tetrasaccharide structure differs from known xanthans in that it is not completely acetylated or pyruvylated. The compounds described, supra, may be polymerized via intracellular or extracellular processes.

The isolation and characterization of the XfGUM operon allows the skilled artisan to determine what substances act as inhibitors or activators of the operon as a whole, or the individual members of the operon. As indicated supra, the xanthan like products of the invention have various industrial uses, and determination of activators and/or agonists permits the artisan to increase production of the materials under appropriate conditions, such as laboratory culture of Xylella fastidiosa, or transformed or transfected cells which produce the xanthan like compound. “In the field” at it were, it is desirable to inhibit or antagonize X. fastidiosa, and the operon and isolated nucleic acid molecules of the invention permits the artisan to identify materials which are useful in inhibiting the organism.

Other facets of the invention will be clear to the skilled artisan, and need not be elaborated here.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, it being recognized that various modifications are possible within the scope of the invention. 

1. An isolated nucleic acid molecule which comprises a nucleotide sequence which encodes the amino acid sequence which is set forth at SEQ ID NO:
 11. 2. The isolated nucleic acid molecule of claim 1, which encodes all of the amino acid sequences which are set forth at SEQ ID NOS: 11, 12, 13, 14, 15, 16, 17, 18, and
 19. 3. The isolated nucleic acid molecule of claim 1, comprising the nucleotide sequences set forth at SEQ ID NO:
 1. 4. The isolated nucleic acid molecule of claim 1 comprising the nucleotide sequence of SEQ ID NO:
 10. 5. An expression vector comprising the isolated nucleic acid molecule of claim 1, operably linked to a promoter.
 6. An expression vector comprising the isolated nucleic acid molecule of claim 2, operably linked to a promoter.
 7. A recombinant cell transformed or transfected with the isolated nucleic acid molecule of claim
 1. 8. A recombinant cell transformed or transfected the isolated nucleic acid molecule of claim
 2. 9. A recombinant cell transformed or transfected the expression vector of claim
 5. 10. A recombinant cell transformed or transfected the expression vector of claim
 6. 